1. Field of the Invention
This invention relates to geological structural modeling of subsurface rock formations, particularly subsurface formation comprising caves (i.e., karst systems).
2. Background Art
While hydrocarbon reservoirs are more often found in porous rocks, karst carbonate reservoirs are important in certain regions in the world, such as China, Middle East, and Russia. For example, the Tarim basin in China contains many oil-rich carbonate fields, in which the oil is contained in caves that formed after dissolution of carbonate rocks by water. The Tarim field is one of the five giant Chinese oilfields with the significant potential for oil production growth in the coming decades.
Unlike conventional porous rock reservoirs, karst systems comprise caves connected with various types of conduits, such as channels and fractures (see FIG. 1A). Therefore, while fluid flows in porous rocks are mostly affected by fluid viscosities and pressure differentials, fluid flows in karst systems are governed mostly by gravity and mass conservation, i.e. similar to open channel flows. Accordingly, conventional dynamic simulators (based on Darcy's law) for porous rocks cannot be directly applied to simulate karst systems.
Due to open channel flow and variable geometries of the caves and conduits, general oil-water contact (OWC) in the region often does not define the boundary of the oil and water. As shown in FIG. 1B, some of the water may be trapped in caves above the general OWC, while some oils may be trapped in caves below the general OWC.
Therefore, dynamic simulation of fluid flows that have been developed for conventional reservoirs based on porous rock models cannot be directly applied to karst systems. In order to simulate flow flows in karst systems, one would need new types of dynamic simulators.
Karst systems have been studied for some time by geologists, speleologists, hydrologists. Many publications covering geomorphological studies, karst creation mechanisms, and bi-phasic flow considerations exist, such as P. Popov et al., “multiscale modeling and simulations of flows in naturally fractured karst reservoirs,” Communications in Computer Physics, 2009, Vol. 6, No. 1, pp. 162-184; and Xiaolong Peng et al., “A New Darcy-Stokes Model for Cavity-Fractured Reservoirs,” 2007, SPE 106751. These publications often concern single-phase flow models. In addition, many fresh water reservoirs have been exploited for potable water. Such water reservoirs are often contained in karst systems. The dynamic simulation of these water reservoirs involves two phases: water and air.
Although dynamic simulators for karst systems are known, there remains a need for better dynamic simulators for karst systems, including simulators for karst systems containing more than two phases.